Correcting transverse scattering in a multi-emitter CT scanner

ABSTRACT

A multi-emitter computed tomography scanner is disclosed, including a plurality of x-ray emitter/detector arrangement pairs arranged offset at an angle to one another. In at least one embodiment, the detector arrangements of the pairs are designed to be energy selective.

PRIORITY STATEMENT

The present application hereby claims priority under 35 U.S.C. §119 onGerman patent application number DE 10 2007 052 448.1 filed Nov. 2,2007, the entire contents of which is hereby incorporated herein byreference.

FIELD

Embodiments of the present invention generally relate to multi-emittercomputed tomography scanners. In at least one embodiment, such scannersinclude a plurality of x-ray emitter/detector pairs, the optical axes ofwhich are offset at an angle to one another.

BACKGROUND

Even though more than two x-ray emitter/detector pairs can be providedin such a computed tomography scanner, the following backgrounddescription is intended to be limited to systems which have two x-rayemitters. Such systems, which are also referred to as dual-source CTsystems, were developed only a few years ago and have substantialadvantages over systems with only one x-ray source. Using a dual-sourcecomputed tomography scanner (DSCT scanner), it is possible to takeimages at a significantly higher speed. It is even possible to obtainimages of the highest quality and with the most accurate details ofweakly or irregularly beating hearts, and at the same time lower theradiation dose.

As has been mentioned previously, two x-ray emitter/detector pairs arearranged with an angular offset of 90° in the case of a dual-source CTsystem. Here, there is a problem in that transversely scattered x-rayradiation is respectively incident on the crossed detector. There are anumber of approaches for correction, some of which are based on modelingand others detect the signal of the transverse scattering inmeasurements and then correct these in a preprocessing step. This makesit possible to reduce as far as possible the signal of the transversescattering. However, the quantum noise of the scattered signal remainsin the data to be evaluated.

SUMMARY

In at least one embodiment of the present invention, a multi-emittercomputed tomography scanner is disclosed, in which the contrast-to-noiseratio in the output signals is improved by extensive suppression oftransverse scattered signals.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following text, the invention will be explained in more detailbased on the description of an example embodiment in conjunction withthe appended drawings, in which:

FIG. 1 shows a very much simplified design of part of a dual-sourcecomputed tomography scanner, and

FIG. 2 shows a spectrum of transverse scattering compared to thespectrum of the primary radiation in a dual-emitter CT scanner.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

Various example embodiments will now be described more fully withreference to the accompanying drawings in which only some exampleembodiments are shown. Specific structural and functional detailsdisclosed herein are merely representative for purposes of describingexample embodiments. The present invention, however, may be embodied inmany alternate forms and should not be construed as limited to only theexample embodiments set forth herein.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the present invention to the particularforms disclosed. On the contrary, example embodiments are to cover allmodifications, equivalents, and alternatives falling within the scope ofthe invention. Like numbers refer to like elements throughout thedescription of the figures.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments of thepresent invention. As used herein, the term “and/or,” includes any andall combinations of one or more of the associated listed items.

It will be understood that when an element is referred to as being“connected,” or “coupled,” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected,” or “directly coupled,” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between,” versus “directly between,” “adjacent,” versus“directly adjacent,” etc.).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments of the invention. As used herein, the singular forms “a,”“an,” and “the,” are intended to include the plural forms as well,unless the context clearly indicates otherwise. As used herein, theterms “and/or” and “at least one of” include any and all combinations ofone or more of the associated listed items. It will be furtherunderstood that the terms “comprises,” “comprising,” “includes,” and/or“including,” when used herein, specify the presence of stated features,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

Spatially relative terms, such as “beneath”, “below”, “lower”, “above”,“upper”, and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, term such as “below” can encompass both anorientation of above and below. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein are interpreted accordingly.

Although the terms first, second, etc. may be used herein to describevarious elements, components, regions, layers and/or sections, it shouldbe understood that these elements, components, regions, layers and/orsections should not be limited by these terms. These terms are used onlyto distinguish one element, component, region, layer, or section fromanother region, layer, or section. Thus, a first element, component,region, layer, or section discussed below could be termed a secondelement, component, region, layer, or section without departing from theteachings of the present invention.

FIG. 1 shows the arrangement of two x-ray emitter/detector pairs 1 a, 2a; 1 b, 2 b, which are arranged offset at an angle of 90° with respectto one another.

A measurement object 4, such as a patient, is arranged in the crossingregion of the beam paths of the two x-ray emitters 1 a, 1 b. The outputradiation of the x-ray emitter 1 a passes through the measurement object4 and is detected by the detector 2 a, lying on the optical axis of thex-ray emitter 1 a. The same holds for the x-ray emitter/detector pair 1b, 2 b. The measurement signals of the two detectors 2 a, 2 b areconditioned in the downstream signal processing units 3 a and 3 b, as isexplained in more detail below, and are processed further for imaging inevaluation electronics (neither described nor illustrated here).

The arrangement of a detector 2 a, 3 a proper, evaluation electronics 3a, 3 b arranged downstream thereof and possibly a filter 5 arranged inthe beam path between the x-ray beams and the detector will be referredto as a detector arrangement in the following text.

In addition to the beams respectively transmitted through themeasurement object, there is also incident, by way of example, on thedetector 2 a x-ray radiation which is emitted by the x-ray emitter 1 band scattered at the measurement object 4 with a scattering angle θ ofapproximately 90°. This transverse scattered radiation proportioncorrupts the measurement signal in the respectively crossed detector.

The energy of the transversely scattered wave can be calculated asfollows:

Compton scattering present here results in the following Compton-shift:

Δλ=λ−λ′=λ_(c)·(1+cos(θ)),

where λ_(c) is the Compton wavelength, θ=90° is the scattering angle,and λ and λ′ are the wavelengths of the incident and scattered waves.Using the relationship

${\lambda = \frac{h \cdot c}{E}},$

where h is Planck's constant and c is the speed of light in a vacuum,the energy of the scattered wave can be calculated using

$E_{out} = {\frac{E_{in}}{1 + {\frac{\lambda_{c}}{h \cdot c} \cdot E_{in}}}.}$

FIG. 2 shows the spectrum 6 of the primary radiation which istransmitted through a 200 mm thick absorber for an example case, wherethe tube voltage of an x-ray emitter is 120 kV. Next to this on theleft, toward lower energies, is the spectrum of the transversescattering 7.

It can be seen that these two spectra 6, 7 hardly overlap. In theillustrated examples, this results, for a tube voltage of 120 keV and aninvariant mass energy of E_(in)=70 keV, in a transversely scattered waveenergy E_(out) of approximately 0.61·E_(in). Thus, it is possible toseparate the respective primary signals and scattered signals intodifferent energy bins. If, as shown in FIG. 1 there is now arranged onlyin the beam path of the emitter/detector pair 1 a, 2 a, a filter 5 whichonly transmits radiation with an energy which is significantly higherthan the energy of the transversely scattered wave (in the example shownin FIG. 2, radiation with an energy of ≧50 kev), then the scatteredsignal is eliminated as far as possible from the measurement signal.

By way of example, such a filter 5 can comprise a radiation-transmissivesubstrate with a tin coating, a tin foil or a tin plate. The quantumnoise of the transverse scattered signal, which undergoes the sameCompton shift, is naturally also eliminated along with the transversescattered signal.

In place of an energy-selective filter arranged in the beam path, thetransverse scattered signals with the associated quantum noise can beseparated from the primary signals using the method described in thefollowing.

Thus, according to an example embodiment of the invention, it islikewise possible to use a quanta-counting detector to suppress thetransverse scattered radiation and its quantum noise. A quanta-countingdetector suitable for computed tomography imaging has a cadmiumtelluride layer as a sensor face. In this case, it is a directlyconverting semiconductor, which directly converts incident x-rayradiation into an electron flow. This flow is linear with respect to thenumber of incident quanta and can be detected using appropriatemeasurement electronics in an energy-resolved manner, that is to say itcan be subdivided into a number of energy bins.

Evaluation electronics downstream of the detector can separate apredetermined low-energy energy bin, which contains the energy spectrumof the transverse scattered radiation, from a higher-energy energy binand disregard the former bin in the subsequent further processing.Consequently, both the transverse scattered signal and the associatedquantum noise of the transverse scattered signal are likewiseeliminated, and the signal-to-noise ratio of the signal to be processedby the evaluation electronics is improved overall.

Although the described example embodiments referred to a dual-emitter CTscanner, it is obvious that the present invention is not limited to thisnumber of emitter/detector pairs; rather it extends to CT systems havingmore than two emitter/detector pairs.

Further, elements and/or features of different example embodiments maybe combined with each other and/or substituted for each other within thescope of this disclosure and appended claims.

Example embodiments being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the present invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1. A multi-emitter computed tomography scanner comprising: a pluralityof x-ray emitter/detector arrangement pairs arranged offset at an angleto one another, detector arrangements of the pairs being designed to beenergy selective.
 2. The multi-emitter computed tomography scanner asclaimed in claim 1, wherein the detector arrangements are useable tofilter out certain energy bins of the incident x-ray radiation.
 3. Themulti-emitter computed tomography scanner as claimed in claim 2, whereina filter is part of the detector arrangements and is arranged upstreamin an incident beam paths of the detector arrangements.
 4. Themulti-emitter computed tomography scanner as claimed in claim 3, whereinthe filter comprises at least one of a part-permeable layer of tin, atin foil and a tin plate, to mostly absorb low energy x-ray quanta. 5.The multi-emitter computed tomography scanner as claimed in claim 1,wherein detectors of the detector arrangements are directly converting,quanta-counting semiconductor detectors which convert the x-ray beamsinto an electron flow which is linear with respect to the number ofincident quanta and which is detected in an energy-resolved manner bydownstream measurement electronics.
 6. The multi-emitter computedtomography scanner as claimed in claim 5, wherein the electron flow isdetected in a number of energy bins.
 7. The multi-emitter computedtomography scanner as claimed in claim 1, wherein the plurality of x-rayemitter/detector arrangement pairs includes only two x-ray emitterdetector arrangement pairs, arranged offset at an angle of 90° withrespect to one another.
 8. The multi-emitter computed tomography scanneras claimed in claim 6, wherein, in further processing of the detectorsignals in the evaluation circuits downstream of the detectors in thedetector arrangements, signals, identified as transverse scatteredsignals, in the low-energy energy bins are not taken into account. 9.The multi-emitter computed tomography scanner as claimed in claim 1,wherein detectors of the detector arrangements have a cadmium telluridelayer for detecting x-ray quanta.
 10. The multi-emitter computedtomography scanner as claimed in claim 2, wherein detectors of thedetector arrangements are directly converting, quanta-countingsemiconductor detectors which convert the x-ray beams into an electronflow which is linear with respect to the number of incident quanta andwhich is detected in an energy-resolved manner by downstream measurementelectronics.
 11. The multi-emitter computed tomography scanner asclaimed in claim 10, wherein the electron flow is detected in a numberof energy bins.
 12. The multi-emitter computed tomography scanner asclaimed in claim 7, wherein, in further processing of the detectorsignals in the evaluation circuits downstream of the detectors in thedetector arrangements, signals, identified as transverse scatteredsignals, in the low-energy energy bins are not taken into account. 13.The multi-emitter computed tomography scanner as claimed in claim 2,wherein detectors of the detector arrangements have a cadmium telluridelayer for detecting x-ray quanta.
 14. The multi-emitter computedtomography scanner as claimed in claim 2, wherein the plurality of x-rayemitter/detector arrangement pairs includes only two x-ray emitterdetector arrangement pairs, arranged offset at an angle of 90° withrespect to one another.
 15. The multi-emitter computed tomographyscanner as claimed in claim 6, wherein the plurality of x-rayemitter/detector arrangement pairs includes only two x-ray emitterdetector arrangement pairs, arranged offset at an angle of 90° withrespect to one another.